The present invention relates to an image forming apparatus. More specifically, the present invention relates to an image forming apparatus capable of forming a color image.
A conventional image forming apparatus includes a printer of an electro-photography type, a facsimile, a copier, and an MFP (a multi function peripheral or product) having three functions of the printer, the facsimile, and the copier. Especially, there is a conventional image forming apparatus capable of forming a color image.
The conventional image forming apparatus capable of forming a color image is provided with a plurality of image forming units corresponding to developer (toner) in a plurality of colors. In the conventional image forming apparatus, a developer image (a toner image) corresponding to each color is formed on a photosensitive drum disposed in each of the image forming units. Then, the developer images are overlapped (transferred) on a transfer medium (a sheet or a transfer belt), thereby forming a color image.
In the conventional image forming apparatus, it is necessary to properly adjust color balance to obtain a color image with high quality. To this end, the conventional image forming apparatus may be configured such that an energy amount of a static latent image forming unit and an energy amount of a developing unit are controlled to adjust a density of each color, thereby adjusting the color balance (Refer to Patent Reference).
Patent Reference: Japanese Patent Publication No. 2004-258281
In the conventional image forming apparatus described above, an energy amount of light to be irradiated on the photosensitive drum is defined as the energy amount of the static latent image forming unit. The energy amount of light is represented with, for example, an exposure time of an exposure unit such as an LED (Light emitting diode) and a laser light source or a drive power of the exposure unit. In the following description, the energy amount of the static latent image forming unit is referred to as an exposure light energy. Further, an energy amount represented with a developing voltage, a supply voltage, a charging voltage, or a combination thereof is defined as the energy amount of the developing unit. In the following description, the energy amount of the developing unit is referred to as a developing energy.
In the conventional image forming apparatus described above, when the color balance is adjusted, first, a density detection test pattern is printed (transferred) on the transfer medium (the transfer belt). A density detection unit (a density sensor) measures a density of the test pattern, so that one of the exposure light energy or the developing energy is controlled according to the density of the test pattern.
In this case, the exposure light energy is controlled through adding a correction value of a light amount to a printing condition. The developing energy is controlled through adding a correction value of the developing voltage to a printing condition. After one of the exposure light energy or the developing energy is controlled, a density detection test pattern with a different density is printed (transferred) on the transfer medium (the transfer belt). Then, the density detection unit (a density sensor) measures a density of the test pattern, so that the other of the exposure light energy or the developing energy is controlled according to the density of the test pattern. Through the process described above, the conventional image forming apparatus adjusts the color balance.
In general, the conventional image forming apparatus is capable of selectively printing a monochrome image and a color image. In the following description, a mode of printing a monochrome image is referred to as a monochrome mode, and a mode of printing a color image is referred to as a color mode. Note that a monochrome image mostly includes an image formed of black developer, and may include an image formed of single developer other than black developer.
In the conventional image forming apparatus, the monochrome mode and the color mode are switched as follows. In the monochrome mode, it is assumed that the monochrome image is formed of black developer.
In order to switch between the monochrome mode and the color mode, the conventional image forming apparatus is provided with an up-down mechanism for selectively lifting or lowering each of the image forming units. The up-down mechanism is provided as a separation contacting mechanism for selectively lifting or lowering each of the image forming units, so that each of the image forming units selectively contacts with or is separated from a transfer unit.
When the conventional image forming apparatus performs a printing operation in the monochrome mode, the up-down mechanism is operated to contact only the image forming unit corresponding to black developer (referred to as a black image forming unit) with the transfer unit, and separate the image forming units corresponding to other colors (referred to as other image forming units) from the transfer unit.
When the conventional image forming apparatus performs the printing operation in the color mode, the up-down mechanism is operated to contact all of the image forming units with the transfer unit. Through the process described above, the conventional image forming apparatus switches between the monochrome mode and the color mode.
When the conventional image forming apparatus performs the printing operation in the monochrome mode and the color mode, an image printed in a color (for example, black) commonly used in the monochrome mode and the color mode may have different densities as described below. The color commonly used in the monochrome mode and the color mode is referred to as a common color.
When the conventional image forming apparatus performs the printing operation in the monochrome mode, only the image forming unit corresponding to the common color (for example, black) contacts with the transfer unit, so that an image in the common color formed on the photosensitive drum is transferred to the transfer medium. When the conventional image forming apparatus performs the printing operation in the color mode, all of the image forming units contact with the transfer unit, so that images in all colors formed on the photosensitive drums are transferred to the transfer medium.
When the image forming units corresponding to the colors other than the common color are arranged on a downstream side of the image forming unit corresponding to the common color, the image in the common color may contact with the image forming units corresponding to the colors other than the common color. In other words, when the conventional image forming apparatus performs the printing operation in the color mode, the image in the common color may contact with the number of the image forming units different from that of the image forming units when the conventional image forming apparatus performs the printing operation in the monochrome mode. As a result, the image in the common color (for example, black) may have different densities in the monochrome mode and the color mode.
In view of the problems described above, an object of the present invention is to provide an image forming apparatus capable of solving the problems of the conventional image forming apparatus. In the present invention, it is possible to form an image in the common color (for example, black) with a same density when the image forming apparatus performs a printing operation in the monochrome mode and the color mode.
Further objects and advantages of the invention will be apparent from the following description of the invention.